The purpose of this contract is to provide genetic monitoring resource to assure the genetic integrity of inbred stock at all production facilities producing F344/N rats and B6C3F1 hybrid mice to the toxicology research and testing program of the NTP as well as the animals supplied to the testing facilities. Up to 15 designated loci will be monitored for each strain or hybrid by electrophoresis of erythrocyte lysates, kidney homogenates and serum proteins. Parental strains of B6C3F1 hybrid mice, hybrid mice and F344 rats from each of the foundation colonies, and production centers will be evaluated for genetic integrity at periodic intervals. Genetic integrity of inbred rodents will be evaluated by skin grafts also. In addition, frozen kidneys from B6C3F1 hybrid mice received for chronic studies at the testing facilities will be subjected to isoenzyme analysis by electrophoresis to assure genetic integrity of mice used in chronic studies. Fischer 344/N rats and B6C3F1 hybrid mice are the selected experimental animal species for evaluation of chemicals for toxicologic and carcinogenic properties under DTRT/NTP. The genetic integrity of the test animals is essential for developing reliable and accurate research data in animal experiments. Constant monitoring for biochemical genetic variants of rederived as well as production stock and test animals will ensure that data from NTP animal studies will be collected from genetically homogenous rats and mice. Genetic homogeneity of test animals will be essential for program wide comparison of the background incidence of tumors and lesions between testing laboratories, between chemicals and within the same laboratory overtime.